Branched conjugated diene copolymer, rubber composition and pneumatic tire

ABSTRACT

The present invention relates to a branched conjugated diene copolymer which is useful for preparing a rubber composition, having high uniformity of a temperature dependence curve of a viscoelasticity tanδ, for a tire, a rubber composition comprising the copolymer, and a pneumatic tire produced using the rubber composition for a tire. The branched conjugated diene copolymer is composed of monomer components comprising a branched conjugated diene compound represented by a general formula (1): 
     
       
         
         
             
             
         
       
     
     wherein R 1  represents an aliphatic hydrocarbon group having 6 to 11 carbon atoms, and an aromatic vinyl compound represented by a general formula (2): 
     
       
         
         
             
             
         
       
     
     wherein R 2  represents an aromatic hydrocarbon group having 6 to 10 carbon atoms, and R 5  represents a hydrogen atom or the like, and a copolymerization ratio (m) of the aromatic vinyl compound (2) is 45% by mass or more.

TECHNICAL FIELD

The present invention relates to a branched conjugated diene copolymer,a rubber composition comprising the copolymer, and a pneumatic tireproduced using the rubber composition.

BACKGROUND OF THE INVENTION

A high grip performance is demanded for tires as a basic property. It isgenerally known, as a method for improving a grip performance of tires,to blend in a rubber composition for a tire a rubber having a high glasstransition temperature (Tg) (for example, one having Tg of −25° C. ormore) or carbon black having a large surface area. In order to make aglass transition temperature high, for example, there is a method ofincreasing a styrene content in a polymer. However, when acopolymerization ratio of an aromatic vinyl compound such as styrenebecomes as high as 45% by mass or more or when a molecular weight (Mw)of a polymer becomes as high as more than 500000, there is a problem,for example, that a styrene chain distribution in a polymer increases,and a temperature dependence curve (a curve obtained by plotting tanδvalues at each temperature when the temperature is changed) of aviscoelasticity tanδ of a rubber composition for a tire prepared usingsuch a polymer shows multiple peaks.

For vehicles, especially general cars such as passenger cars, low fuelconsumption, namely improvement in rolling resistance is demanded fromenvironmental point of view in addition to a grip performance, inparticular a wet grip performance as a basic performance. Aviscoelasticity tanδ of a rubber composition is an index for such wetgrip performance and low fuel consumption. Namely, a tanδ at 0° C. is anindex for wet grip performance, and the higher the tanδ is, the better abraking efficiency is. A tanδ at 60° C. is an index for rollingresistance, and the lower the tanδ is, the better the fuel consumptionis.

In addition, for vehicles for racing, a high grip performance isdemanded as a basic performance. A viscoelasticity tanδ of a rubbercomposition is an index for such grip performance. Namely, a tanδ in atemperature range of from 20° to 100° C., particularly from 30° to 45°C., is an index for grip performance within such a temperature range,and the higher the tanδ is, the better a braking efficiency is.

Therefore, if a temperature dependence curve of tanδ can be controlledso that its shape becomes as uniform as possible without multiple peaks,it is possible to provide a rubber composition for a tire exhibitingintended desired characteristics (for example, a rubber composition fora tire having improved grip performance within a specific temperaturerange, or the like). Therefore, in the development of a polymer for atire rubber, control of a temperature dependence curve of tanδ is animportant issue to be addressed.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a branchedconjugated diene copolymer which is useful for preparing a rubbercomposition, having high uniformity of a temperature dependence curve ofa viscoelasticity tanδ, for a tire, a rubber composition comprising thecopolymer, and a pneumatic tire produced using the rubber compositionfor a tire.

The inventors of the present invention have made intensive studies inorder to solve the above-mentioned problem and as a result, have foundthat by using a specific branched conjugated diene copolymer as a rubbercomponent, in which a copolymerization ratio of the aromatic vinylcompound is as high as 45% by mass or more based on the whole monomersand a specific branched conjugated diene monomer is contained as a partof monomer components constituting the copolymer, uniformity of atemperature dependence curve of a viscoelasticity tanδ of the obtainedrubber composition can be enhanced even if the copolymerization ratio ofthe aromatic vinyl compound is high. Thus, further studies have beenmade and the present invention has been completed.

Namely, the present invention relates to a branched conjugated dienecopolymer composed of monomer components comprising a branchedconjugated diene compound represented by a general formula (1):

wherein R¹ represents an aliphatic hydrocarbon group having 6 to 11carbon atoms, and an aromatic vinyl compound represented by a generalformula (2):

wherein R² represents an aromatic hydrocarbon group having 6 to 10carbon atoms, and R⁵ represents a hydrogen atom or an alkyl group having1 to 3 carbon atoms,wherein a copolymerization ratio (m) of the aromatic vinyl compound (2)is 45% by mass or more.

It is preferable that a glass transition temperature of theabove-mentioned branched conjugated diene copolymer is −25° C. or more.

It is preferable that a glass transition temperature of theabove-mentioned branched conjugated diene copolymer is −10° C. or more.

It is preferable that a weight-average molecular weight of theabove-mentioned branched conjugated diene copolymer is more than 100000.

It is preferable that the above-mentioned monomer components furthercomprise a conjugated diene compound represented by a general formula(3):

wherein R³ and R⁴ are the same or different, and each represents ahydrogen atom, an aliphatic hydrocarbon group having 1 to 3 carbonatoms, or a halogen atom.

It is preferable that in the above-mentioned branched conjugated dienecopolymer, a copolymerization ratio (1) of the branched conjugated dienecompound (1) is from 1 to 54% by mass, a copolymerization ratio (m) ofthe aromatic vinyl compound (2) is from 45 to 99% by mass, and acopolymerization ratio (n) of the conjugated diene compound (3) is from0 to 54% by mass.

It is preferable that the branched conjugated diene compound (1) ismyrcene and/or farnesene.

It is preferable that the aromatic vinyl compound (2) is one or moreselected from the group consisting of styrene, α-methylstyrene,α-vinylnaphthalene and β-vinylnaphthalene.

It is preferable that the above-mentioned conjugated diene compound (3)is 1,3-butadiene and/or isoprene.

Further, the present invention relates to a rubber compositioncomprising the above-mentioned branched conjugated diene copolymer as arubber component, in which a half width at half maximum of aviscoelasticity tanδ defined by the following equation is 40 or less.

Half width at half maximum=(Higher temperature at half height of tanδpeak)−(Tanδ peak temperature)

Furthermore, the present invention relates to a pneumatic tire producedusing the above-mentioned rubber composition.

According to the present invention, by using a specific branchedconjugated diene monomer as a part of monomer components constitutingthe copolymer, uniformity of a temperature dependence curve of aviscoelasticity tanδ of the rubber composition prepared using thecopolymer can be enhanced even if the copolymerization ratio of thearomatic vinyl compound monomer is as high as 45% by mass or more. Byusing such a copolymer, it is possible to provide a rubber compositionfor a tire exhibiting a desired performance, for example, a rubbercomposition for a tire having improved wet grip performance and fuelconsumption performance or a rubber composition for a tire (for example,a rubber composition for a tire for racing cars) having improved gripperformance within a specific temperature range (for example, within aspecific temperature range of from 20° to 100° C., preferably from 30°to 45° C.).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing temperature dependence curves of aviscoelasticity tanδ of vulcanized rubber compositions 1 to 3 (Example1, Example 2 and Comparative Example 1).

FIG. 2 is a graph showing temperature dependence curves of aviscoelasticity tanδ of vulcanized rubber compositions 4 to 9 (Examples3 to 7 and Comparative Example 2).

DETAILED DESCRIPTION

As mentioned above, the first aspect of the present invention relates tothe branched conjugated diene copolymer which can enhance uniformity ofa temperature dependence curve of a viscoelasticity tanδ.

The second aspect relates to the branched conjugated diene copolymerwhich can enhance uniformity of a temperature dependence curve of aviscoelasticity tanδ as shown representatively in FIG. 1 and can improvewet grip performance and fuel efficiency by utilizing the enhanceduniformity.

The third aspect relates to the branched conjugated diene copolymerwhich can enhance uniformity of a temperature dependence curve of aviscoelasticity tanδ as shown representatively in FIG. 2 and can improvegrip performance within a specific temperature range (for example,within a specific temperature range of from 20° to 100° C., preferablyfrom 30° to 45° C.).

The branched conjugated diene copolymer of the present invention is onecomposed of the monomer components comprising the branched conjugateddiene compound (1) and at least 45% by mass of the aromatic vinylcompound (2) or one composed of the monomer components furthercomprising the conjugated diene compound (3).

(Copolymer)

The copolymerization ratios of the branched conjugated diene compound(1), the aromatic vinyl compound (2) and the conjugated diene compound(3) as monomers in the branched conjugated diene copolymer of thepresent invention are explained below.

The copolymerization ratio (1) of the branched conjugated diene compound(1) is not limited particularly as long as it is from 1 to 55% by mass.The copolymerization ratio is preferably 2% by mass or more, morepreferably 2.5% by mass or more, further preferably 5% by mass or more.If the copolymerization ratio is less than 1% by mass, there is atendency that a sufficient effect for improvement to eliminate multiplepeaks in the viscoelasticity tanδ curve cannot be obtained. Thecopolymerization ratio is preferably 20% by mass or less, morepreferably 15% by mass or less. This is because when the compound isblended in a copolymerization ratio of as much as 20% by mass, there isa tendency that a sufficient effect resulting from the blending of thebranched conjugated diene compound (1) can be obtained.

The copolymerization ratio (n) of the aromatic vinyl compound (2) is notlimited particularly as long as it is from 45 to 99% by mass. Thecopolymerization ratio is preferably 46% by mass or more, morepreferably 47% by mass or more, more preferably 48% by mass or more,more preferably 49% by mass or more, further preferably 50% by mass ormore. If the copolymerization ratio is less than 45% by mass, there is atendency that such a copolymerization ratio is not an extent causing aproblem with multiple peaks in the viscoelasticity tanδ curve, and aneffect of copolymerizing the branched conjugated diene compound (1) forimprovement to eliminate multiple peaks is decreased. Thecopolymerization ratio is preferably 70% by mass or less, morepreferably 60% by mass or less. If the copolymerization ratio is morethan 70% by mass, there is a concern such that the tanδ (60° C.) becomeshigh and sufficient fuel consumption performance is not obtained.

The copolymerization ratio (m) of the conjugated diene compound (3) isnot limited particularly as long as it is from 0 to 54% by mass. Thecopolymerization ratio is preferably 2% by mass or more, more preferably5% by mass or more, and is preferably 50% by mass or less, morepreferably 45% by mass or less.

With respect to the above-mentioned copolymerization ratios, in the caseof the branched conjugated diene copolymer of the present inventioncomprising only the compounds (1) and (2), when the copolymerizationratio of one of them is determined within the above-mentioned range, thecopolymerization ratio of another compound is also determinedaccordingly. In the case of the branched conjugated diene copolymer ofthe present invention comprising only the compounds (1) to (3), when thecopolymerization ratios of any two compounds are determined within theabove-mentioned ranges, the copolymerization ratio of the remainingcompound is also determined accordingly.

<Branched Conjugated Diene Compound>

In the branched conjugated diene compound (1), examples of the aliphatichydrocarbon group having 6 to 11 carbon atoms are those having a normalstructure such as hexyl, heptyl, octyl, nonyl, decyl and undecyl,isomers and/or unsaturated groups thereof, and derivatives thereof (forexample, halides, hydroxides, and the like). Preferred examples are4-methyl-3-pentenyl group, 4,8-dimethyl-nona-3,7-dienyl group, and thelike, and derivatives thereof.

Examples of the branched conjugated diene compound (1) are myrcene,farnesene, and the like.

In particular, β-myrcene (7-methyl-3-methyleneocta-1,6-diene) having thefollowing structure is preferred as myrcene.

In particular, (E)-β-farnesene(7,11-dimethyl-3-methylene-1,6,10-dodecatriene) having the followingstructure is preferred as farnesene.

The branched conjugated diene compounds (1) can be used alone or can beused in combination of two or more thereof.

<Aromatic Vinyl Compound>

In the aromatic vinyl compound (2), examples of the aromatic hydrocarbongroup having 6 to 10 carbon atoms are phenyl, benzyl, phenethyl, tolyl,xylyl, naphthyl, and the like. The substitution position of methyl onthe benzene ring of tolyl includes any of ortho, meta and parapositions, and the substitution position of methyl in xylyl alsoincludes any of optional substitution positions. Among these groups,preferred are phenyl, tolyl and naphthyl. Examples of the alkyl grouphaving 1 to 3 carbon atoms are methyl, ethyl, n-propyl and isopropyl,and among these, methyl is preferred.

Examples of the preferred aromatic vinyl compound (2) are styrene,α-methylstyrene, α-vinylnaphthalene and β-vinylnaphthalene.

The aromatic vinyl compounds (2) can be used alone or can be used incombination of two or more thereof.

<Conjugated Diene Compound>

In the conjugated diene compound (3), examples of the aliphatichydrocarbon group having 1 to 3 carbon atoms are methyl, ethyl,n-propyl, isopropyl, and the like, and among these, methyl is preferred.Examples of the halogen atom are fluorine, chlorine, bromine and iodine,and among these, chlorine is preferred.

Examples of the conjugated diene compound (3) are 1,3-butadiene,isoprene, 2,3-dimethyl-1,3-butadiene, and the like, and among these,1,3-butadiene and isoprene are preferred.

The conjugated diene compounds (3) can be used alone or can be used incombination of two or more thereof.

<Glass Transition Temperature>

The glass transition temperature (Tg) of the branched conjugated dienecopolymer of the present invention is preferably −25° C. or more. If itis less than −25° C., there is a tendency that sufficient gripperformance cannot be obtained. Tg is preferably 35° C. or less. When Tgexceeds 35° C., there is a tendency that the rubber composition becomebrittle and processability is lowered.

In the second aspect of the present invention, it is necessary to makethe tanδ (0° C.) high for obtaining sufficient wet grip performance. Inthis case, Tg is preferably −25° C. or more, more preferably −20° C. ormore. When Tg is less than −25° C., there is a tendency that the tanδ(0° C.) decreases and sufficient wet grip performance cannot beobtained. Tg is preferably 15° C. or less, more preferably 10° C. orless. When Tg is more than 15° C., there is a tendency that the tanδ(60° C.) becomes high and sufficient fuel consumption performance is notobtained. Therefore, since the branched conjugated diene copolymerhaving Tg of from −25° C. to 15° C. exhibits sufficient wet gripperformance and low fuel consumption, it is suitable as a rubbercomponent for a tire for general vehicles (for example, passenger cars(PC) and trucks and buses (TB)).

In the third aspect of the present invention, for exhibiting sufficientgrip performance within a specific temperature range between 20° C. and100° C., preferably 30° and 45° C., Tg is preferably −10° C. or more,more preferably −5° C. or more. When Tg is less than −10° C., there is atendency that sufficient grip performance cannot be obtained. Tg ispreferably 35° C. or less, more preferably 25° C. or less. When Tgexceeds 35° C., there is a tendency that the rubber composition becomebrittle and processability is lowered. Therefore, since the branchedconjugated diene copolymer of the present invention exhibits sufficientgrip performance, it is suitable for uses as a rubber component for atire for racing cars.

Tg of the branched conjugated diene copolymer tends to become lower asthe amount of the contained conjugated diene compound (3), for example,high-cis butadiene prepared using a transition-metal catalyst isincreased, and tends to become higher as the amount of the containedaromatic vinyl compound (2) such as styrene prepared using an anionicpolymerization catalyst is increased.

In addition, Tg can be adjusted by an amount of a polar compound to beused for preparing the branched conjugated diene copolymer. Namely, whenthe amount of polar compound is increased, there is a tendency that theamount of vinyl in a butadiene structure increases and, accordingly, Tgincreases, and when the amount of polar compound is decreased, there isa tendency that the amount of vinyl in a butadiene structure decreasesand, accordingly, Tg decreases.

<Half Width at Half Maximum of Viscoelasticity tanδ>

In the present invention, the half width at half maximum of aviscoelasticity tanδ is a value obtained based on a peak shape of thetemperature dependence curve of a viscoelasticity tanδ of the rubbercomposition, and concretely is obtained by the following equation.

Half width at half maximum=(Higher temperature at half height of tanδpeak)−(Tanδ peak temperature)

Here, the temperature dependence curve of a viscoelasticity tanδ is “acurve obtained by plotting tanδ values of the rubber composition at eachtemperature when the temperature is changed”. In the present invention,the tanδ is a value measured at a dynamic strain amplitude of 1% at afrequency of 10 Hz using a spectrometer (Model: VES-F1112 available fromUeshima Seisakusho Co., Ltd.).

In the present invention, the half width at half maximum ofviscoelasticity tanδ is not more than 40, preferably not more than 30,more preferably not more than 20. When the half width at half maximumexceeds 40, there is a tendency that the peak height of the tanδdecreases and sufficient grip performance cannot be obtained.

In addition, from the viewpoint of sufficient wet grip performance andfuel consumption performance, the half width at half maximum ofviscoelasticity tanδ is not more than 40, preferably not more than 30,more preferably not more than 20. When the half width at half maximumexceeds 40, there is a tendency that a balance between the wet gripperformance and the fuel consumption performance is lowered andperformance of a tire is not achieved sufficiently.

The half width at half maximum generally has a correlation with theuniformity of a temperature dependence curve of a viscoelasticity tanδ,and it is known that the smaller the half width at half maximum is, thehigher the uniformity of the curve is. Here, “uniformity” means that thecurve shows a single peak (a concept as opposed to multiple peaks),which means that the tanδ peak becomes relatively higher. Therefore, bycontrolling the half width at half maximum so as to make it smaller, theuniformity of a temperature dependence curve of a viscoelasticity tanδcan be increased and the tanδ peak can be made high, thereby making itpossible to provide a rubber composition having desired characteristics,for example, a rubber composition having improved wet grip performanceand fuel consumption performance or a rubber composition having improvedgrip performance, especially grip performance within a specifictemperature range of from 20° C. to 100° C., preferably from 30° to 45°C.

<Tanδ>

A tanδ value is an index for grip performance, etc.

In the second aspect of the present invention, for example, tanδ at 0°C. (tanδ (0° C.)) is an index for wet grip performance, and a largertanδ value is regarded as good for braking property. The value ispreferably 0.4 or more, further preferably 0.6 or more. When it is lessthan 0.4, there is a tendency that sufficient wet grip performancecannot be obtained. An upper limit of tanδ (0° C.) is not restrictedparticularly. Tanδ at 60° C. (tanδ (60° C.)) is an index for rollingresistance, and a smaller tanδ value is regarded as good for giving goodfuel consumption performance. The tanδ (60° C.) value is preferably 0.4or less, further preferably 0.35 or less. When it is more than 0.4,there is a tendency that sufficient fuel efficiency is not achieved.

In the third aspect of the present invention, a larger tanδ value withinthe above-mentioned specific temperature range of from 20° C. to 100°C., preferably from 30° to 45° C. is good for good braking property. Thetanδ value is preferably 0.5 or more, further preferably 0.6 or more.When it is less than 0.5, there is a tendency that sufficient gripperformance cannot be obtained. An upper limit of the tanδ is notlimited particularly.

In the present invention, the peak value of tanδ is not limitedparticularly, and is preferably 0.4 or more, further preferably 0.5 ormore. When it is less than 0.4, there is a tendency that sufficient gripperformance at each temperature cannot be attained. An upper limit ofthe peak value of tanδ is not restricted particularly.

<Molecular Weight>

The weight-average molecular weight (Mw) of the branched conjugateddiene copolymer of the present invention is not limited particularly aslong as it is 100000 or more. The weight-average molecular weight ispreferably 500000 or more. When Mw is less than 100000, there is atendency that the polymer is in a liquid form having no rubberelasticity. Mw is preferably 3000000 or less. When Mw exceeds 3000000,there is a tendency that the polymer is in a solid form having no rubberelasticity.

In the branched conjugated diene copolymer, a ratio of Mw to anumber-average molecular weight (Mn), namely Mw/Mn is preferably 10.0 orless, more preferably 5.0 or less. When the Mw/Mn exceeds 10.0, there isa tendency that the polymer becomes a softened product having no rubberelasticity. A lower limit of the Mw/ Mn is not limited particularly, andwhen it is 1.0 or more, no problem arises.

<Mooney Viscosity>

A Mooney viscosity ML₁₊₄ (130° C.) of the branched conjugated dienecopolymer of the present invention is generally preferably 25 or more,more preferably 30 or more. When the Mooney viscosity is less than 25,the copolymer tends to have fluidity. The Mooney viscosity is preferably160 or less, more preferably 150 or less, further preferably 100 orless, further preferably 60 or less. When the Mooney viscosity exceeds160, there is a tendency that large amounts of a softening agent andprocessing aid are necessary at the time of processing.

The Mooney viscosity ML₁₊₄ (130° C.) of the branched conjugated dienecopolymer of the present invention is characterized by being lower ascompared with that of a copolymer which is obtained by replacing thebranched conjugated diene compound (1) constituting the copolymer withthe conjugated diene compound (3) and has the same molecular weight.Therefore, the branched conjugated diene copolymer is useful forimproving processability when preparing the rubber composition.

<Preparation Method>

The branched conjugated diene copolymer of the present invention can beobtained by copolymerizing the branched conjugated diene compound (1),the aromatic vinyl compound (2), and if desired, the conjugated dienecompound (3).

In such a copolymerization process, an order of copolymerization ofmonomers is not limited particularly. For example, all monomers may besubjected to random copolymerization simultaneously, or after previouslycopolymerizing specific monomer or monomers (for example, only thebranched conjugated diene compound (1), only the aromatic vinyl compound(2), only the conjugated diene compound (3) or two kinds of monomersarbitrarily selected from these), the remaining monomers or monomer maybe added and copolymerized, or each monomer may be previouslycopolymerized respectively, and then subjected to blockcopolymerization.

Such copolymerization can be carried out by a usual method, for example,by anionic polymerization reaction, coordination polymerizationreaction, or the like.

A polymerization method is not limited particularly, and any of asolution polymerization method, an emulsion polymerization method, a gasphase polymerization method and a bulk polymerization method can beused. Among these, a solution polymerization method is preferred. Thepolymerization may be carried out batchwise or continuously.

<Anionic Polymerization>

Anionic polymerization can be carried out in a proper solvent in thepresence of an anionic initiator. As an anionic initiator, any of usualones can be used suitably, and examples of such an anionic initiator areorganolithium compounds having a general formula RLix (R is analiphatic, aromatic or alicyclic group having one or more carbon atoms,x is an integer of 1 to 20). Examples of proper organolithium compoundsare methyllithium, ethyllithium, n-butyllithium, sec-butyllithium,tert-butyllithium, phenyllithium and naphthyllithium. Preferredorganolithium compounds are sec-butyllithium and tert-butyllithium.Anionic initiators can be used alone or can be used in a mixture of twoor more thereof. An amount of a polymerization initiator for anionicpolymerization is not limited particularly, and it is preferable to use,for example, in an amount of preferably from about 0.05 mmol to 35 mmol,more preferably from about 0.05 mmol to 0.2 mmol per 100 g of allmonomers to be subjected to polymerization.

As a solvent to be used for the anionic polymerization, any of solventscan be used suitably as long as they neither inactivate the anionicinitiator nor stop the polymerization reaction, and any of polarsolvents and nonpolar solvents can be used. Examples of polar solventsare ether solvents such as tetrahydrofuran, and examples of nonpolarsolvents are chain hydrocarbons such as hexane, heptane, octane andpentane, cyclic hydrocarbons such as cyclohexane, aromatic hydrocarbonssuch as benzene, toluene and xylene, and the like. These solvents can beused alone or can be used in a mixture of two or more thereof.

It is further preferable to carry out the anionic polymerization in thepresence of a polar compound. Examples of polar compounds are dimethylether, diethyl ether, ethyl methyl ether, ethyl propyl ether,tetrahydrofuran, dioxane, diphenyl ether, tripropylamine, tributylamine,trimethylamine, triethylamine, N,N,N′,N′-tetramethylethylenediamine(TMEDA), and the like. Polar compounds can be used alone or can be usedin a mixture of two or more thereof. The polar compound is useful forreducing the content of 1,2-structure in the micro structure ofbutadiene portion. The amount of polar compound varies depending on kindthereof and the polymerization conditions, and a molar ratio thereof tothe anionic initiator (polar compound/anionic initiator) is preferably0.1 or more. When the molar ratio of the polar compound to the anionicinitiator (polar compound/anionic initiator) is less than 0.1, there isa tendency that an effect of using the polar compound for controllingthe micro structure is not sufficient.

The reaction temperature of the anionic polymerization is not limitedparticularly as long as the reaction advances properly, and usually ispreferably from −10° to 100° C., more preferably from 25° to 70° C. Inaddition, the reaction time varies depending on charging amounts,reaction temperature and other conditions, and usually, for example,about 3 hours is sufficient.

The above-mentioned anionic polymerization can be terminated by adding areaction inhibitor to be usually used in this field. Examples of thereaction inhibitor are polar solvents having an active proton such asalcohols, for example, methanol, ethanol and isopropanol or acetic acid,a mixture thereof, or a mixture of the polar solvents with nonpolarsolvents such as hexane and cyclohexane. A sufficient amount of reactioninhibitor is usually an equimolar amount or two-fold molar amount withthe anionic initiator.

After the polymerization reaction, the branched conjugated dienecopolymer can be separated from the polymerization solution easily byremoving the solvent by a usual method or by pouring the polymerizationsolution in an alcohol of an amount equal to or more than the amount ofpolymerization solution and precipitating the branched conjugated dienecopolymer.

<Coordination Polymerization>

The coordination polymerization can be carried out using a coordinationpolymerization initiator instead of the anionic initiator in the anionicpolymerization. Any of usual coordination polymerization initiators canbe suitably used, and examples thereof are catalysts that are transitionmetal-containing compounds such as lanthanoid compounds, titaniumcompounds, cobalt compounds and nickel compounds. In addition, ifdesired, an aluminum compound or a boron compound can be used as aco-catalyst.

The lanthanoid compound is not limited particularly as long as itcontains any of elements (lanthanoids) of atomic numbers 57 to 71, andamong these lanthanoids, neodymium is preferred particularly. Examplesof the lanthanoid compounds are carboxylates, β-diketone complexes,alkoxides, phosphates, phosphites, halides and the like of theseelements. Among these, from the viewpoint of easy handling,carboxylates, alkoxides, and β-diketone complexes are preferred.Examples of the titanium compounds are titanium-containing compoundshaving a cyclopentadienyl group, an indenyl group, a substitutedcyclopentadienyl group, or a substituted indenyl group and also having 1to 3 substituents selected from halogen, an alkoxysilyl group and analkyl group, and preferred are compounds having one alkoxysilyl groupfrom the viewpoint of catalytic activity. Examples of the cobaltcompounds are halides, carboxylates, β-diketone complexes, organic basecomplexes, organic phosphine complexes, and the like of cobalt. Examplesof the nickel compounds are halides, carboxylates, β-diketone complexes,organic base complexes, and the like of nickel. Catalysts to be used asa coordination polymerization initiator can be used alone or can be usedin combination of two or more thereof. An amount of a catalyst to beused as a polymerization initiator for the coordination polymerizationis not limited particularly, and for example, a preferred amount thereofis the same as the amount of the catalyst for the anionicpolymerization.

Examples of the aluminum compounds to be used as a co-catalyst areorganic aluminoxanes, halogenated organoaluminum compounds,organoaluminum compounds, hydrogenated organoaluminum compounds, and thelike. Examples of the organic aluminoxanes are alkyl aluminoxanes (suchas methyl aluminoxane, ethyl aluminoxane, propyl aluminoxane, butylaluminoxane, isobutyl aluminoxane, octyl aluminoxane, and hexylaluminoxane); examples of the halogenated organoaluminum compounds arehalogenated alkyl aluminum compounds (such as dimethyl aluminumchloride, diethyl aluminum chloride, methyl aluminum dichloride, andethyl aluminum dichloride); examples of the organoaluminum compounds arealkyl aluminum compounds (such as trimethylaluminum, triethylaluminum,triisopropylaluminum, and triisobutylaluminum); and examples of thehydrogenated organoaluminum compounds are hydrogenated alkyl aluminumcompounds (such as diethylaluminum hydride, and diisobutylaluminumhydride). Examples of the boron compounds are compounds having anionspecies such as tetraphenylborate, tetrakis(pentafluorophenyl)borate,and (3,5-bistrifluoromethylphenyl)borate. These co-catalysts can also beused alone or can be used in combination of two or more thereof.

In the coordination polymerization, the solvents and the polar compoundsexplained in the anionic polymerization can be used similarly. Inaddition, the reaction time and the reaction temperature are the same asthose explained in the anionic polymerization. Termination of thepolymerization reaction and separation of the branched conjugated dienecopolymer can also be carried out in the same manner as in the anionicpolymerization.

The weight-average molecular weight (Mw) of the branched conjugateddiene copolymer can be controlled by adjusting the amounts of branchedconjugated diene and other monomers to be charged at the polymerization.For example, by increasing the ratio of all monomers to the anionicpolymerization catalyst, Mw can be increased, and by decreasing theratio, Mw can be decreased. The same is true also for the number-averagemolecular weight (Mn) of the branched conjugated diene copolymer.

Tg of the branched conjugated diene copolymer can be controlled byadjusting the amount of aromatic vinyl compound (2) to be charged at thepolymerization. For example, by increasing the copolymerization ratio ofaromatic vinyl compound (2), Tg can be made high, and on the contrary,by decreasing the copolymerization ratio of aromatic vinyl compound (2),Tg can be made low.

The Mooney viscosity of the branched conjugated diene copolymer can becontrolled by adjusting the amount of branched conjugated diene monomer(1) to be charged at the polymerization. For example, by decreasing theamount of branched conjugated diene compound (1), the Mooney viscosityis increased, and on the contrary, by increasing the amount, the Mooneyviscosity is decreased.

(Rubber Composition)

By blending other components which are usually used in the field ofrubber industry with the thus obtained branched conjugated dienecopolymer of the present invention, a rubber composition for a tire canbe prepared.

Examples of the components to be blended in the rubber composition ofthe present invention are rubber components other than the branchedconjugated diene copolymer, a filler, a silane coupling agent, and thelike.

In the rubber composition for a tire of the present invention, theamount of the branched conjugated diene copolymer in the rubbercomponents is about 10% by mass or more, preferably 20% by mass or more.When the amount of the branched conjugated diene copolymer is less than10% by mass, there is a tendency that the effect on the viscoelasticitytanδ curve of the rubber composition by blending the branched conjugateddiene copolymer is decreased. Meanwhile, an upper limit of the amount ofthe branched conjugated diene copolymer is not limited particularly.

In the present invention, examples of the rubber components to be usedtogether with the branched conjugated diene copolymer are diene rubberssuch as a natural rubber (NR), an isoprene rubber (IR), a butadienerubber (BR), a styrene-butadiene rubber (SBR), a styrene-isoprene rubber(SIR), a styrene-isoprene-butadiene rubber (SIBR), an ethylene propylenediene rubber (EPDM), a chloroprene rubber (CR), anacrylonitrile-butadiene rubber (NBR), a butyl rubber (IIR), and thelike. These diene rubbers may be used alone or may be used incombination of two or more thereof. Among these, it is preferable to useNR, BR, or SBR for the reason that a well-balanced grip performance andabrasion resistance can be obtained in combination use with the branchedconjugated diene copolymer, and it is more preferable to use NR. NR isnot limited particularly, and those commonly used for production oftires can be used. Examples thereof are SIR20, RSS#3, TSR20, and thelike.

Examples of the filler are carbon black, silica, and the like which arecommonly used in this field.

Carbon blacks which are used generally in production of tires can beused, and examples thereof are SAF, ISAF, HAF, FF, FEF, GPF, and thelike. These carbon blacks can be used alone or can be used incombination of two or more thereof. The nitrogen adsorption specificsurface area (N₂SA) of carbon black is not less than about 80 m²/g,preferably not less than about 110 m²/g. When N₂SA is less than 80 m²/g,both of grip performance and abrasion resistance tend to be lowered.When N₂SA is less than 110 m²/g, an effect of using the branchedconjugated diene copolymer for improving processability tends to bedecreased. N₂SA of carbon black is not more than about 270 m²/g,preferably not more than about 260 m²/g. When N₂SA of carbon black ismore than 270 m²/g, dispersibility of carbon black tends to bedecreased. N₂SA of carbon black is determined according to A method ofJIS K 6217.

A blending amount of carbon black is not less than about 1 part by mass,preferably not less than about 3 parts by mass based on 100 parts bymass of the rubber components. When the blending amount of carbon blackis less than 1 part by mass, abrasion resistance tends to be lowered.The blending amount of carbon black is not more than about 200 parts bymass, more preferably not more than 150 parts by mass. When the blendingamount of carbon black exceeds 200 parts by mass, processability tendsto be lowered.

As silica, for example, silica (silicic anhydride) prepared by a drymethod and silica (hydrous silicate) prepared by a wet method areexemplified. Among these, silica prepared by a wet method is preferredfor the reason that there are many surface silanol groups and manyreaction points with a silane coupling agent. N₂SA of silica is not lessthan about 50 m²/g, preferably not less than about 80 m²/g. When N₂SA isless than 50 m²/g, there is a tendency that a reinforcing effect issmall and abrasion resistance is decreased. N₂SA of silica is not morethan about 300 m²/g, preferably not more than about 250 m²/g. When N₂SAis more than 300 m²/g, there is a tendency that dispersibility of silicais decreased and processability is lowered. N₂SA of silica is determinedby BET method according to ASTM D3037-93.

A blending amount of silica is not less than about 1 part by mass,preferably not less than about 10 parts by mass based on 100 parts bymass of the rubber components. When the blending amount of silica isless than 1 part by mass, there is a tendency that abrasion resistanceis not sufficient. The blending amount of silica is not more than about150 parts by mass, more preferably not more than 100 parts by mass. Whenthe blending amount of carbon black exceeds 150 parts by mass, there isa tendency that dispersibility of silica is decreased and processabilityis lowered.

It is preferable that the rubber composition comprises a silane couplingagent. As the silane coupling agent, a silane coupling agent which hasbeen well-known can be used. Examples thereof are sulfide silanecoupling agents such as bis(3-triethoxysilylpropyl)tetrasulfide,bis(2-triethoxysilylethyl)tetrasulfide,bis(4-triethoxysilylbutyl)tetrasulfide,bis(3-trimethoxysilylpropyl)tetrasulfide,bis(2-trimethoxysilylethyl)tetrasulfide,bis(4-trimethoxysilylbutyl)tetrasulfide,bis(3-triethoxysilylpropyl)trisulfide,bis(2-triethoxysilylethyl)trisulfide,bis(4-triethoxysilylbutyl)trisulfide,bis(3-trimethoxysilylpropyl)trisulfide,bis(2-trimethoxysilylethyl)trisulfide,bis(4-trimethoxysilylbutyl)trisulfide,bis(3-triethoxysilylpropyl)disulfide,bis(2-triethoxysilylethyl)disulfide,bis(4-triethoxysilylbutyl)disulfide,bis(3-trimethoxysilylpropyl)disulfide,bis(2-trimethoxysilylethyl)disulfide,bis(4-trimethoxysilylbutyl)disulfide,3-trimethoxysilylpropyl-N,N-dimethylthiocarbamoyl tetrasulfide,3-triethoxysilylpropyl-N,N-dimethylthiocarbamoyl tetrasulfide,2-triethoxysilylethyl-N,N-dimethylthiocarbamoyl tetrasulfide,2-trimethoxysilylethyl-N,N-dimethylthiocarbamoyl tetrasulfide,3-trimethoxysilylpropylbenzothiazolyl tetrasulfide,3-triethoxysilylpropylbenzothiazole tetrasulfide, and3-trimethoxysilylpropyl methacrylate monosulfide; mercapto silanecoupling agents such as 3-mercaptopropyltrimethoxysilane,3-mercaptopropyltriethoxysilane, 2-mercaptoethyltrimethoxysilane and2-mercaptoethyltriethoxysilane; vinyl silane coupling agents such asvinyltriethoxysilane and vinyltrimethoxysilane; amino silane couplingagents such as 3-aminopropyltriethoxysilane,3-aminopropyltrimethoxysilane, 3-(2-aminoethyl)aminopropyltriethoxysilane and 3-(2-aminoethyl)aminopropyltrimethoxysilane; glycidoxysilane coupling agents such asγ-glycidoxypropyltriethoxysilane, γ-glycidoxypropyltrimethoxysilane,γ-glycidoxypropylmethyldiethoxysilane andγ-glycidoxypropylmethyldimethoxysilane; nitro silane coupling agentssuch as 3-nitropropyltrimethoxysilane and 3-nitropropyltriethoxysilane;and chloro silane coupling agents such as3-chloropropyltrimethoxysilane, 3-chloropropyltriethoxysilane,2-chloroethyltrimethoxysilane and 2-chloroethyltriethoxysilane, and thelike. These silane coupling agents can be used alone, or can be used incombination of two or more thereof. From the viewpoint of goodprocessability, it is preferable that among these,bis(3-triethoxysilylpropyl)tetrasulfide orbis(3-triethoxysilylpropyl)disulfide is contained in the rubbercomposition.

When the silane coupling agent is contained, the blending amount thereofis preferably not less than 1 part by mass, more preferably not lessthan 2 parts by mass based on 100 parts by mass of silica. When theamount of silane coupling agent is less than 1 part by mass, there is atendency that a sufficient improving effect of dispersibility cannot beobtained. The amount of silane coupling agent is preferably not morethan 20 parts by mass, more preferably not more than 15 parts by mass.When the amount of silane coupling agent exceeds 20 parts by mass, thereis a tendency that a sufficient coupling effect cannot be obtained and areinforcing property is decreased.

In addition to the above-mentioned components, compounding agents whichhave been used in the field of rubber industry, for example, an otherreinforcing filler, an antioxidant, an oil, a wax, a vulcanizing agentsuch as sulfur, a vulcanization accelerator, a vulcanization aid, andthe like can be properly blended to the rubber composition of thepresent invention.

The thus obtained rubber composition of the present invention can beused as various parts for tires, for example, can be suitably usedespecially for a tire tread since wet grip performance and low fuelconsumption can be improved to a high level and grip performance withina specific temperature range can be improved.

(Pneumatic Tire)

The rubber composition of the present invention can be used forproduction of tires and can be formed into tires by a usual method.Namely, a mixture obtained by optionally blending the above-mentionedcomponents according to necessity is subjected to kneading, extrusionprocessing to a shape of each part of a tire at an unvulcanized stage,and molding on a tire molding machine by a usual method, thus forming anunvulcanized tire. A tire can be obtained by heating and compressingthis unvulcanized tire in a vulcanizer, and by introducing air in thetire, a pneumatic tire can be obtained.

Herein, Mw and Mn are measured using a gel permeation chromatograph(GPC), and are converted based on standard polystyrene.

A glass transition temperature (Tg) is measured with a differentialscanning calorimeter (DSC).

A Mooney viscosity is measured in accordance with JIS K 6300.

A range simply indicated by, for example, “1 to 99% by mass” isconstrued so as to include the figures at both ends.

EXAMPLE

The present invention is explained by means of Examples, but is notlimited to the Examples.

Various chemicals used for synthesis of diene copolymers and preparationof rubber compositions in Examples and Comparative Examples arecollectively shown below. The various chemicals were subjected topurification according to necessity by a usual method.

<Various Chemicals Used for Synthesis of Copolymers>

Cyclohexane: Cyclohexane available from Kanto Chemical Industry Co.,Ltd.

Isopropanol: Isopropanol available from Kanto Chemical Industry Co.,Ltd.

Branched conjugated diene compound: β-Myrcene available from Wako PureChemical Industries, Ltd.

Aromatic vinyl compound: Styrene available from Wako Pure ChemicalIndustries, Ltd.

Conjugated diene compound: 1,3-Butadiene available from TakachihoChemical Industrial Co., Ltd.

Polar compound: Tetrahydrofuran (THF) available from Wako Pure ChemicalIndustries, Ltd.

<Various Chemicals Used for Preparation of Rubber Composition>

Copolymer: Those synthesized in accordance with the description of thisspecification

Carbon black: DIABLACK A (N110, Nitrogen adsorption specific surfacearea (N₂SA): 130 m²/g) available from Cabot Corporation

Oil: PROCESS X-260 available from JX Nippon Oil & Energy Corporation

Stearic acid: Stearic acid available from NOF CORPORATION

Zinc oxide: Zinc White Grade 2 available from Mitsui Mining & SmeltingCo., Ltd.

Sulfur: Powdered sulfur available from Tsurumi Chemical Industry Co.,Ltd.

Vulcanization accelerator: NOCCELER NS(N-tert-butyl-2-benzothiazolylsulfenamide) available from Ouchi ShinkoChemical Industrial Co., Ltd.

(Examples 1 and 2 and Comparative Example 1) Example 1 (1) (Synthesis ofCopolymer 1)

Into a 3-liter pressure resistant stainless steel vessel having beensubjected to drying and replacement with nitrogen, 1500 ml ofcyclohexane, 10 g of myrcene, 50 g of styrene, 40 g of butadiene and 3ml of THF were poured, and further, 0.4 mmol of n-butyllithium (n-BuLi)was added thereto, followed by 3-hour polymerization reaction at 40° C.After three hours had elapsed, 0.44 ml of 1M isopropanol/hexane solutionwas added dropwise to terminate the reaction. The obtainedpolymerization solution was subjected to blast drying to remove thesolvent, followed by drying under reduced pressure at an inner pressureof 0.1 kPa or less at a temperature of 50° C. until a constant weight isreached. Thus, 100 g (dry mass) of Copolymer 1 was obtained. The degreeof polymerization (percentage of dry mass/charged amount) was nearly100%.

(2) (Preparation of Unvulcanized Rubber Composition 1)

The Copolymer 1 obtained above and the above-mentioned various chemicalsfor preparation of a rubber composition (except insoluble sulfur andvulcanization accelerator) were kneaded at 150° C. for five minutes in aBanbury mixer in accordance with the formulation shown in Table 2, and akneaded product was obtained. Sulfur and vulcanization accelerator wereadded to the kneaded product, followed by 12-minute kneading at 170° C.using an open roll to obtain Unvulcanized Rubber Composition 1.

(3) (Preparation of Vulcanized Rubber Composition 1)

The Unvulcanized Rubber Composition 1 obtained in (2) above wassubjected to 20-minute press-vulcanization at 170° C. to obtainVulcanized Rubber Composition 1.

Example 2 (1) (Synthesis of Copolymer 2)

Processing was carried out in the same manner as in (1) of Example 1except that the amounts of myrcene and butadiene were changed to 20 gand 30 g, respectively, to obtain 100 g of Copolymer 2. The degree ofpolymerization was nearly 100%.

(2) (Preparation of Unvulcanized Rubber Composition 2)

Processing was carried out in the same manner as in (2) of Example 1except that Copolymer 2 was used instead of Copolymer 1, to obtainUnvulcanized Rubber Composition 2.

(3) (Preparation of Vulcanized Rubber Composition 2)

Unvulcanized Rubber Composition 2 obtained in (2) above was subjected toprocessing in the same manner as in (3) of Example 1 to obtainVulcanized Rubber Composition 2.

Comparative Example 1 (1) (Synthesis of Copolymer 3)

Processing was carried out in the same manner as in (1) of Example 1except that 50 g of styrene and 50 g of butadiene were used instead ofusing 10 g of myrcene, 50 g of styrene, and 40 g of butadiene, to obtain100 g of Copolymer 3. The degree of polymerization was nearly 100%.

(2) (Preparation of Unvulcanized Rubber Composition 3)

Processing was carried out in the same manner as in (2) of Example 1except that Copolymer 3 was used instead of Copolymer 1, to obtainUnvulcanized Rubber Composition 3.

(3) (Preparation of Vulcanized Rubber Composition 3)

Unvulcanized Rubber Composition 3 obtained in (2) above was subjected toprocessing in the same manner as in (3) of Example 1 to obtainVulcanized Rubber Composition 3.

With respect to the obtained Copolymers 1 to 3, the following tests werecarried out. The results are shown in Table 1.

(Measurement of Micro Structure (Vinyl Amount (mol %), Styrene Amount(mass %)))

The micro structure was measured with an apparatus ADVANCE II Seriesavailable from BRUKER BIOSPIN K.K.

(Measurement of Weight-Average Molecular Weight (Mw), Number-AverageMolecular Weight (Mn))

Mw and Mn were measured with an apparatus GPC-8000 Series available fromTOSO CORPORATION and a differential refractometer as a detector, andwere converted based on standard polystyrene.

(Measurement of Glass Transition Temperature (Tg))

Measurement was carried out using a differential scanning calorimeter(apparatus DSC Q200 Series available from TA Instruments, Japan) at aheat-up rate of 10° C/min from an initial temperature of −150° C. to afinal temperature of 150° C. to calculate Tg.

TABLE 1 Example Com. Ex. 1 2 1 Copolymer 1 2 3 Charging amount Branchedconjugated diene 10 20 0 compound (% by mass) Aromatic vinyl compound 5050 50 (% by mass) Conjugated diene compound 40 30 50 (% by mass) Polarcompound (ml) 3 3 3 Results Yield (%) 100 100 100 Vinyl amount (mol %)33 34 32 Styrene amount (% by mass) 48 47 46 Number-average molecularweight 600000 610000 600000 (Mn) Weight-average molecular weight 660000670000 670000 (Mw) Molecular weight distribution 1.1 1.1 1.1 (Mw/Mn)Glass transition temperature (Tg) −23° C. −24° C. −26° C.

The following tests were carried out using the obtained VulcanizedRubber Compositions 1 to 3 or using Test Tires 1 to 3 with a treadportion formed from Unvulcanized Rubber Compositions 1 to 3 (size:195/65R15, vulcanization conditions: 170° C., 20 min). The results areshown in Table 2.

(Half Width at Half Maximum of Viscoelasticity tanδ)

A change in tanδ value of the Vulcanized Rubber Compositions 1 to 3relative to a temperature change was measured at a dynamic strainamplitude of 1% at a frequency of 10 Hz using a spectrometer (Model:VES-F1112 available from Ueshima Seisakusho Co., Ltd.). The results areshown in FIG. 1. From FIG. 1, half widths at half maximum ofviscoelasticity tanδ of Examples and Comparative Example were obtained.

(Rolling Resistance)

Test Tires 1 to 3 were run using a rim (15x6JJ) at an inner pressure of230 kPa at a load of 3.43 kN at a speed of 80 km/h, and rollingresistances thereof were measured with a rolling resistance tester. Therolling resistance is indicated by an index on the assumption that therolling resistance of Comparative Example 1 is 100. The smaller theindex is, the better the rolling resistance is.

(Wet Grip Performance)

A vehicle equipped with Test Tires 1 to 3 (rim: 15x6JJ, inner pressure:230 kPa) were run on a wet asphalt road at an initial speed of 100 km/h,and a braking distance was measured. The results are indicated by anindex shown by the following equation. The larger the figure is, thebetter the wet skid performance (wet grip performance) is. The index wasobtained by the following equation.

Wet skid performance=(Braking distance of Comparative Example1)÷(Braking distance of each Example or each Comparative Example)

(Tire Balance)

A tire balance was evaluated by a driver using the vehicle used in theabove-mentioned evaluation of wet grip performance, and the results arerepresented by an index obtained in accordance with on the followingcriteria. The larger the figure is, the better the tire balance is.Namely, the tire balance was evaluated using the following ten itemsunder the condition that a maximum evaluation point of each item is 20and the point of Comparative Example 1 is 10. Therefore, the maximumevaluation point is 200, and the total point of Comparative Example 1 is100.

-   -   (1) Response near N    -   (2) Response at steering    -   (3) Grip level at turning    -   (4) Grip level at lane change    -   (5) Vehicle stability at outlet of curve    -   (6) Delay in follow-up action for yawing    -   (7) Linearity of yawing    -   (8) Yaw gain    -   (9) Self-straightening property    -   (10) Steering stability against rough road surface

(Processability)

A test piece of a given size was prepared from the above unvulcanizedrubber composition, and a Mooney viscosity ML₁₊₁₀ (130° C.) thereof wasmeasured using a Mooney viscosity tester in accordance with JIS K 6300“Test Method of Unvulcanized Rubber”. The test piece was pre-heated to130° C. for one minute and under this temperature condition, a largerotor was rotated and after a lapse of ten minutes, the Mooney viscosityML₁₊₁₀ (130° C.) was measured. The smaller the Mooney viscosity is, thebetter the processability is.

TABLE 2 Example Com. Ex. 1 2 1 Blending amount (part by mass) Copolymer1 100 Copolymer 2 100 Copolymer 3 100 Carbon black 50 50 50 Oil 30 30 30Stearic acid 2 2 2 Zinc oxide 1.4 1.4 1.4 Sulfur 2 2 2 Vulcanizationaccelerator 2 2 2 Results of evaluation Half width at half maximum oftanδ 40 11 81 Peak height of tanδ 0.72 1.11 0.44 Tanδ (0° C.) 0.65 1.000.42 Tanδ (60° C.) 0.34 0.23 0.40 Rolling resistance 81 56 100 Wet gripperformance 150 250 100 Tire balance 165 182 100 Processability (Mooneyviscosity) 48.2 45.0 53.5

(Examples 3 to 7 and Comparative Example 2) Example 3 (1) (Synthesis ofCopolymer 4)

Into a 3-liter pressure resistant stainless steel vessel having beensubjected to drying and replacement with nitrogen, 1500 ml ofcyclohexane, 10 g of myrcene, 50 g of styrene, 40 g of butadiene and 30ml of THF were poured, and further, 0.4 mmol of n-butyllithium (n-BuLi)was added thereto, followed by 3-hour polymerization reaction at 40° C.After three hours had elapsed, 0.44 ml of 1M isopropanol/hexane solutionwas added dropwise to terminate the reaction. The obtainedpolymerization solution was subjected to blast drying to remove thesolvent, followed by drying under reduced pressure at an inner pressureof 0.1 kPa or less at a temperature of 50° C. until a constant weight isreached. Thus, 100 g (dry mass) of Copolymer 4 was obtained. The degreeof polymerization (percentage of dry mass/charged amount) was nearly100%.

-   -   (2) (Preparation of Unvulcanized Rubber Composition 4)

The obtained Copolymer 4 and the above-mentioned various chemicals forpreparation of a rubber composition (except insoluble sulfur andvulcanization accelerator) were kneaded at 150° C. for five minutes in aBanbury mixer in accordance with the formulation shown in Table 4, and akneaded product was obtained. Sulfur and vulcanization accelerator wereadded to the kneaded product, followed by 12-minute kneading at 170° C.using an open roll to obtain Unvulcanized Rubber Composition 4.

(3) (Preparation of Vulcanized Rubber Composition 4)

The Unvulcanized Rubber Composition 4 obtained in (2) above wassubjected to 20-minute press-vulcanization at 170° C. to obtainVulcanized Rubber Composition 4.

Example 4 (1) (Synthesis of Copolymer 5)

Processing was carried out in the same manner as in (1) of Example 3except that the amounts of myrcene and butadiene were changed to 20 gand 30 g, respectively, to obtain 100 g of Copolymer 5. The degree ofpolymerization was nearly 100%.

(2) (Preparation of Unvulcanized Rubber Composition 5)

Processing was carried out in the same manner as in (2) of Example 3except that Copolymer 5 was used instead of Copolymer 4, to obtainUnvulcanized Rubber Composition 5.

(3) (Preparation of Vulcanized Rubber Composition 5)

Unvulcanized Rubber Composition obtained in (2) above was subjected toprocessing in the same manner as in (3) of Example 3 to obtainVulcanized Rubber Composition 5.

Example 5 (1) (Synthesis of Copolymer 6)

Processing was carried out in the same manner as in (1) of Example 3except that the amounts of myrcene and butadiene were changed to 30 gand 20 g, respectively, to obtain 100 g of Copolymer 6. The degree ofpolymerization was nearly 100%.

(2) (Preparation of Unvulcanized Rubber Composition 6)

Processing was carried out in the same manner as in (2) of Example 3except that Copolymer 6 was used instead of Copolymer 4, to obtainUnvulcanized Rubber Composition 6.

(3) (Preparation of Vulcanized Rubber Composition 6)

Unvulcanized Rubber Composition obtained in (2) above was subjected toprocessing in the same manner as in (3) of Example 3 to obtainVulcanized Rubber Composition 6.

Example 6 (1) (Synthesis of Copolymer 7)

Processing was carried out in the same manner as in (1) of Example 3except that the amounts of myrcene, styrene, and butadiene were changedto 10 g, 60 g, and 30 g, respectively, to obtain 100 g of Copolymer 7.The degree of polymerization was nearly 100%.

(2) (Preparation of Unvulcanized Rubber Composition 7)

Processing was carried out in the same manner as in (2) of Example 3except that Copolymer 7 was used instead of Copolymer 4, to obtainUnvulcanized Rubber Composition 7.

(3) (Preparation of Vulcanized Rubber Composition 7)

Unvulcanized Rubber Composition obtained in (2) above was subjected toprocessing in the same manner as in (3) of Example 3 to obtainVulcanized Rubber Composition 7.

Example 7 (1) (Synthesis of Copolymer 8)

Processing was carried out in the same manner as in (1) of Example 3except that the amounts of myrcene, styrene, and butadiene were changedto 20 g, 60 g, and 20 g, respectively, to obtain 100 g of Copolymer 8.The degree of polymerization was nearly 100%.

(2) (Preparation of Unvulcanized Rubber Composition 8)

Processing was carried out in the same manner as in (2) of Example 3except that Copolymer 8 was used instead of Copolymer 4, to obtainUnvulcanized Rubber Composition 8.

(3) (Preparation of Vulcanized Rubber Composition 8)

Unvulcanized Rubber Composition obtained in (2) above was subjected toprocessing in the same manner as in (3) of Example 3 to obtainVulcanized Rubber Composition 8.

Comparative Example 2 (1) (Synthesis of Copolymer 9)

Processing was carried out in the same manner as in (1) of Example 3except that 50 g of styrene and 50 g of butadiene were used instead of10 g of myrcene, 50 g of styrene, and 40 g of butadiene, to obtain 100 gof Copolymer 9. The degree of polymerization was nearly 100%.

(2) (Preparation of Unvulcanized Rubber Composition 9)

Processing was carried out in the same manner as in (2) of Example 3except that Copolymer 9 was used instead of Copolymer 4, to obtainUnvulcanized Rubber Composition 9.

(3) (Preparation of Vulcanized Rubber Composition 9)

Unvulcanized Rubber Composition obtained in (2) above was subjected toprocessing in the same manner as in (3) of Example 3 to obtainVulcanized Rubber Composition 9.

The same tests as mentioned supra were carried out using the obtainedCopolymers 4 to 9. The results are shown in Table 3.

TABLE 3 Com. Example Ex. 3 4 5 6 7 2 Copolymer 4 5 6 7 8 9 Chargingamount Branched conjugated diene compound 10 20 30 10 20 0 (% by mass)Aromatic vinyl compound (% by mass) 50 50 50 60 60 50 Conjugated dienecompound (% by mass) 40 30 20 30 20 50 Polar compound (ml) 30 30 30 3030 30 Results Yield (%) 100 100 100 100 100 100 Vinyl amount (mol %) 5353 55 54 53 52 Styrene amount (mass %) 48 49 46 61 62 46 Number-averagemolecular weight (Mn) 620000 620000 630000 660000 650000 600000Weight-average molecular weight (Mw) 680000 680000 690000 730000 720000660000 Molecular weight distribution (Mw/Mn) 1.1 1.1 1.1 1.1 1.1 1.1Glass transition temperature (Tg) −7° C. −5° C. −8° C. −5° C. −7° C.−10° C.

The following tests were carried out using the obtained UnvulcanizedRubber Compositions 4 to 9 and Vulcanized Rubber Compositions 4 to 9.The results are shown in Table 4. In addition, the test forprocessability was carried out in the same manner as mentioned supra.

(Half Width at Half Maximum of Viscoelasticity tanδ)

A change in tanδ value of the Vulcanized Rubber Compositions 4 to 9relative to a temperature change was measured at a dynamic strainamplitude of 1% at a frequency of 10 Hz using a spectrometer (Model:VES-F1112 available from Ueshima Seisakusho Co., Ltd.). The results areshown in FIG. 2. From FIG. 2, half widths at half maximum ofviscoelasticity tanδ of Examples and Comparative Example were obtained.

(Grip Performance)

Tires (size: 195/65R15, vulcanization conditions: 170° C., 20 min)having a tread produced using the above-mentioned rubber compositionswere produced, and an in-vehicle running test was carried out on anasphalt circuit course using the produced tires (rim: 15x6JJ, innerpressure: 230 kPa). A test driver evaluated stability of steeringcontrol, and the stability is indicated by FIGS. 1 to 5 on theassumption that the stability of Comparative Example 2 is 1. The largerthe figure is, the higher and the more excellent the grip performanceis.

TABLE 4 Com. Example Ex. 3 4 5 6 7 2 Blending amount (part by mass)Copolymer 4 100 Copolymer 5 100 Copolymer 6 100 Copolymer 7 100Copolymer 8 100 Copolymer 9 100 Carbon black 50 50 50 50 50 50 Oil 30 3030 30 30 30 Stearic acid 2 2 2 2 2 2 Zinc oxide 1.4 1.4 1.4 1.4 1.4 1.4Sulfur 2 2 2 2 2 2 Vulcanization 2 2 2 2 2 2 accelerator Results ofevaluation Half width at 15.1 15.8 19.8 22.4 14.2 63.8 half maximum oftanδ Peak height of 0.79 0.99 1.00 0.77 1.33 0.47 tanδ Grip 2 4 4 2 5 1performance Processability 62 52 47 75 62 77 (Mooney viscosity)

Industrial Applicability

The rubber composition for a tire having enhanced uniformity of atemperature dependence curve of a viscoelasticity tanδcan be prepared byusing the branched conjugated diene copolymer of the present invention.The branched conjugated diene copolymer is useful for preparing a rubbercomposition for a tire exhibiting desired performance.

What is claimed is:
 1. A branched conjugated diene copolymer composed ofmonomer components comprising a branched conjugated diene compoundrepresented by a general formula (1):

wherein R¹ represents an aliphatic hydrocarbon group having 6 to 11carbon atoms, and an aromatic vinyl compound represented by a generalformula (2):

wherein R² represents an aromatic hydrocarbon group having 6 to 10carbon atoms, and R⁵ represents a hydrogen atom or an alkyl group having1 to 3 carbon atoms, wherein a copolymerization ratio (m) of thearomatic vinyl compound (2) is 45% by mass or more.
 2. The branchedconjugated diene copolymer of claim 1, having a glass transitiontemperature of −25° C. or more.
 3. The branched conjugated dienecopolymer of claim 1, having a glass transition temperature of −10° C.or more.
 4. The branched conjugated diene copolymer of claim 1, having aweight-average molecular weight of more than
 100000. 5. The branchedconjugated diene copolymer of claim 1, wherein the monomer componentsfurther comprise a conjugated diene compound represented by a generalformula (3):

wherein R³ and R⁴ are the same or different, and each represents ahydrogen atom, an aliphatic hydrocarbon group having 1 to 3 carbonatoms, or a halogen atom.
 6. The branched conjugated diene copolymer ofclaim 5, wherein a copolymerization ratio (1) of the branched conjugateddiene compound (1) is from 1 to 54% by mass, a copolymerization ratio(m) of the aromatic vinyl compound (2) is from 45 to 99% by mass, and acopolymerization ratio (n) of the conjugated diene compound (3) is from0 to 54% by mass.
 7. The branched conjugated diene copolymer of claim 1,wherein the branched conjugated diene compound (1) is myrcene and/orfarnesene.
 8. The branched conjugated diene copolymer of claim 1,wherein the aromatic vinyl compound (2) is one or more selected from thegroup consisting of styrene, α-methylstyrene, α-vinylnaphthalene andβ-vinylnaphthalene.
 9. The branched conjugated diene copolymer of claim5, wherein the conjugated diene compound (3) is 1,3-butadiene and/orisoprene.
 10. A rubber composition comprising the branched conjugateddiene copolymer of claim 1 as a rubber component, in which a half widthat half maximum of a viscoelasticity tanδ defined by the followingequation is 40 or less.Half width at half maximum=(Higher temperature at half height of tanδpeak)−(Tanδ peak temperature)
 11. A pneumatic tire produced using therubber composition of claim 10.